High resolution digitizers commonly incorporate some sort of filtering to reduce jitter in their output coordinates. Depending upon the nature of the digitizing technique, this may take the form of analog filtering upon signals proportional to the measured coordinates, or digital filtering through the application of numerical methods. The latter type of filtering is especially attractive in a digitizer controlled by a microprocessor. Simple averaging of consecutive measurements is one known method of digital filtering. See, for example, U.S. Pat. No. 4,255,617 issued on Mar. 10, 1981 to Carau, et al.
The reduction of jitter is especially important to a digitizer intended to operate in a polled environment of the following sort. A controller polls devices connected "in series" in an interface "loop". By design, if the device has no new data to report to the controller, it so indicates, and the controller proceeds to poll the next device. This type of operation conserves bandwidth on the loop. However, if the device is a digitizer with jitter, then new coordinates could be reported each time that digitizer were polled, even though the stylus or cursor were held completely stationary! Such a mode of operation would unnecessarily consume the valuable information bandwidth of the loop. It would be desirable to filter the output of such a digitizer to suppress the effects of jitter. What is desired is a digitizer that does not output coordinates unless there has been an actual change in the position of the cursor or stylus. Commercial examples of such digitizers operating with an interface loop of the type described are the Hewlett-Packard model 46087A and model 46088A Digitizing Tablets. These digitizers operate with what is called the HP-HIL, or Hewlett-Packard Human Interface Loop.
Known methods of filtering interfere with a property that may be termed "dynamic accuracy". Dynamic accuracy is the ability of the digitizer to output coordinate values that lie upon the actual path taken by the cursor or stylus, even though the cursor or stylus is moving, perhaps rapidly, and not output coordinates that do not lie upon that path. See the above-mentioned disclosures by Carau and Burgess for descriptions of what can happen when the cursor or stylus is in motion during the process of digitizing. It should be understood that filtering to reduce jitter can aggravate the problem of maintaining good dynamic accuracy. It would be desirable to find a filtering technique to eliminate jitter without sacrificing good dynamic accuracy.
It will also become clear that the technique to be described for the reduction or elimination of jitter is applicable to other types of transducers besides digitizing tablets; e.g., angle transducers. Although it produces only a single "coordinate" and not a pair, the output of an angle transducer is subject to jitter and may include a dynamic accuracy specification. (Rather than requiring that a point lie upon a line, a dynamic accuracy specification for an angle transducer would require a correct correspondence between reported angles and points in time.) The inventive technique may be applied to any stream of coordinates whose values increase or decrease monotonically as a steadily increasing or decreasing input is applied to the transducer.